Many types of semiconductor devices are made using similar manufacturing procedures. A starting substrate, usually a thin wafer of silicon, is doped, masked, and etched through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. Each die on the wafer is given a brief test for full functionality, and the nonfunctional die are mechanically marked or mapped in software. This brief test is only a gross measure of functionality, and does not insure that a die is completely functional or has specifications that would warrant its assembly in a package.
If the wafer has a yield of grossly functional die, it indicates that a good quantity of die from the wafer are likely to be fully operative. The die are separated with a die saw, and the nonfunctional die are scrapped, while the rest are individually encapsulated in plastic packages or mounted in ceramic packages with one die in each package. After the die are packaged they are rigorously electrically tested. Components which turn out to be nonfunctional, or which operate at questionable specifications, are scrapped or devoted to special uses.
Packaging unusable die, only to scrap them after testing, is a waste of time and materials, and is therefore costly. Given the relatively low profit margins of commodity semiconductor components such as dynamic random access memories (DRAMs) and static random access memories (SRAMs), this practice is uneconomical. However, no thorough and cost effective method of testing an unpackaged die is available which would prevent this unnecessary packaging of nonfunctional and marginally functional die. Secondly, the packaging may have other limitations which are aggravated by burn-in stress conditions, so that the packaging becomes a limitation for burn-in testing.
It is proposed that multiple integrated circuit devices be packaged as a single unit, known as a multi chip module (MCM). This can be accomplished with or without conventional lead frames. This creates two problems when using conventional test methods. Firstly, discrete testing is more difficult because a conventional lead frame package is not used. Furthermore, when multiple devices are assembled into a single package, the performance of the package is reduced to that of the die with the lowest performance. Therefore, such dies are tested on an individual basis at probe, using ambient and "hot chuck" test techniques, while still in wafer form. In other words, the ability to presort the individual dice is limited to that obtained through probe testing.
In addition, there is an increased interest in providing parts which are fully characterized prior to packaging. This is desired not only because of the cost of the package, but also because there is demand for multi-chip modules (MCMs), in which multiple parts in die form are tested and assembled into a single unit. While there are various techniques proposed for testing, burning in and characterizing a singulated die, it would be advantageous to be able to "wafer map" the die prior to assembly with as many performance characteristics as possible. Ideally, one would want to be able to map the wafer with full device characterization.
MCMs create a particular need for testing prior to assembly, as contrasted to the economics of testing parts which are discretely packaged as singulated parts. For discretely packaged parts, if the product yield of good parts from preliminary testing to final shipment (probe-to-ship) is, for example, 95%, one would not be particularly concerned with packaging costs for the failed parts, if packaging costs are 10% of the product manufacturing costs. Even where packaging costs are considerably higher, as in ceramic encapsulated parts, testing unpackaged die is economical for discretely packaged parts when the added costs approximates that of cost of packaging divided by yield: ##EQU1## where c=cost
c.sub.die =manufacturing cost of functional die PA1 c.sub.addl KG =additional cost of testing unpackaged die in order to produce known good die
Note that in the case of discretely packaged parts, the cost of the die (C.sub.DIE) is essentially not a factor. This changes in the case of MCMs: ##EQU2## Note that again C.sub.DIE is not a factor in modules having identical part types; however, the equation must be modified to account for varied costs and yields of die in modules with mixed part types.
With MCMs, the cost of packaging a failed part is proportional to the number of die in the module. In the case of a x16 memory array module, where probe-to-ship yield of the die is 95%, the costs are: ##EQU3## so the additional costs of testing for known good die (KGD) may be 16 times the cost of testing an unrepairable module and still be economical. This, of course, is modified by the ability to repair failed modules.
Testing of unpackaged die before packaging into multichip modules would be desirable as it would result in reduced material waste, increased profits, and increased throughput. Using only known good die in MCMs would increase MCM yields significantly.
Testing unpackaged die requires a significant amount of handling. Since the test package must be separated from the die, the temporary packaging may be more complicated than either standard discrete packaging or multichip module (MCM) packaging. The package must be compatible with test and burn-in procedures, while securing the die without damaging the die at the bondpads or elsewhere during the process.
We propose an approach for testing of unpackaged die which utilizes a two piece reusable burn-in/test fixture. The fixture consists of two halves, one of which is a die cavity plate for receiving a semiconductor die as the devices under test (DUT). A die is placed in a cavity in a first half of the fixture, and a die contact member is used to establish contact with bondpads on the die, and to conduct between the bondpads and external connector leads on the fixture.
External connector leads are used, and may be provided in a preferred configuration, such as a DIP (dual inline package) or QFP (quad flat pack) configuration. The fixture establishes electrical contact with a single die and with a burn-in oven, as well as permitting testing of dice in discretely packaged form.
Such a configuration requires that the die bondpads or other contact points be aligned with contacts in the fixture. The fixture must then maintain the die in alignment without damage to the die, and particularly to the bondpads. The fixture is then manipulated through test procedures.
One advantage of the temporary package is that it need not meet the requirements of a conventional package in terms of flexibility of installation; that is, it can be cumbersome and not readily adaptable to an end use other than the test and burn-in equipment. The die is expected to be moved out of the temporary package subsequent to testing, so that the package need only be useful for the various test and burn-in procedures.
Since the temporary package is specifically intended for test and burn-in procedures, it is desired that the package be easy to assemble on a temporary basis, and thereby at least partially facilitate the test and burn-in procedures.
In U.S. Pat. No. 4,899,107, commonly assigned, a reusable burn-in/test fixture for discrete TAB die is taught. The fixture consists of two halves, one of which is a die cavity plate for receiving semiconductor dies as the units under test (UUT); and the other half establishes electrical contact with the dies and with a burn-in oven.
The first half of the test fixture contains cavities in which die are inserted circuit side up. The die will rest on a floating platform. The second half has a rigid high temperature rated substrate, on which are mounted probes for each corresponding die pad. Each of a plurality of probes is connected to an electrical trace on the substrate (similar to a P.C. board) so that each die pad of each die is electrically isolated from one another for high speed functional testing purposes. The probe tips are arranged in an array to accommodate eight or sixteen dies.
The two halves of the test fixture are joined so that each pad on each die aligns with a corresponding probe tip. The test fixture is configured to house groups of 8 or 16 die for maximum efficiency of the functional testers.
There are some testing and related procedures when the parts are singulated. For this reason, it is inconvenient, to retain multiple die in a single test fixture.
Various forms of connections are used to connect the die to a package or, in the case of a multichip module (MCM), to other connections. These include wirebonding, TAB connections, bump bonding directly to substrate, and conductive adhesives.
The bondpads are conductive areas on the face of the die which are used as an interconnect for connecting the circuitry on the die to the outside world. Normally, conductors are bonded to the bondpads, but it is possible to establish electrical contact through the bondpads by biasing conductors against the bondpads without actual bonding.
One of the problems encountered with burn in and full characterization testing of unpackaged die is the physical stress caused by connection of the bondpads to an external connection circuitry. This problem is complicated by the fact that in many die configurations, the bondpads are recessed below the surface level of a passivation layer. The passivation layer is a layer of low eutectic glass, such as BPSG, which is applied to the die in order to protect circuitry on the die. (The term "eutectic" does not, strictly speaking, apply to glass, which is an amorphous fluid; however, the term is used to describe the characteristic of some glasses wherein, as a result of their formulation, they readily flow at a given temperature.)
The ohmic contact between bondpads or test points on a die and a known good die test carrier package has been a matter of interest. It is difficult to achieve and maintain consistent ohmic contact without damaging the bondpads and passivation layer on the die. The design criteria of such contacts is somewhat different from the design criteria of the carrier package.
A prior art apparatus for packaging semiconductor devices includes a carrier tray which accepts a plurality of ceramic type packages, such as DIP (dual in-line package) or QFP (quad flat pack) packages. With the carrier supporting the package, the die is inserted, secured to the package and electrically attached to the package. A metallic lid is supported on the package by a bridge clamp which is clamped to the carrier tray over the package. The bridge clamp, in turn, clamps the lid against the package. The lid is then fused to the package, typically by soldering. The packages are then removed from the carrier tray.